Cast steel frame for gas turbine engine

ABSTRACT

A gas turbine engine comprises a first turbine module, a second turbine module, and a frame interconnecting the first turbine module with the second turbine module. The frame includes a plurality of circumferentially distributed struts extending radially between an inner hub and an outer case, and is formed from a single steel sand casting.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No. 61/747,271 filed Dec. 29, 2012 for “CAST STEEL FRAME FOR GAS TURBINE ENGINE” by Jonathan Ariel Scott and PCT Application No. PCT/US 13/77124 filed Dec. 20, 2013 for “CAST STEEL FRAME FOR GAS TURBINE ENGINE” by Jonathan Ariel Scott.

BACKGROUND

The described subject matter relates generally to gas turbine engines, and more specifically to cases and frames for gas turbine engines.

Gas turbine engines operate according to a continuous-flow, Brayton cycle. A compressor section pressurizes an ambient air stream, fuel is added and the mixture is burned in a combustor section. The combustion products expand through a turbine section where bladed rotors convert thermal energy from the combustion products into mechanical energy for rotating one or more centrally mounted shafts. The shafts, in turn, drive the forward compressor section, thus continuing the cycle. Gas turbine engines are compact and powerful power plants, making them suitable for powering aircraft, heavy equipment, ships and electrical power generators. In power generating applications, the combustion products can also drive a separate power turbine attached to an electrical generator.

Gas turbine engines are supported by frames which typically include one or more struts. The struts connect outer and inner cases and cross a flow passage carrying working gases such as combustion products. Due to the need for the struts to retain their strength at high temperatures, frames used on the turbine side of the engine have been produced using investment cast superalloys. However, casting of superalloys becomes more difficult and expensive as the radial dimension of the frame increases. Increased frame size thus has required the struts to be individually cast along with separate inner and outer cases, which are then individually welded or otherwise bonded. This results in a tradeoff between engine size and manufacturing effort.

SUMMARY

A gas turbine engine comprises a first turbine module, a second turbine module, and a frame interconnecting the first turbine module with the second turbine module. The frame comprises a single, unified steel casting which includes a plurality of circumferentially distributed struts extending radially between an inner hub and an outer case.

A turbine exhaust case (TEC) assembly for a gas turbine engine comprises a frame and a fairing assembly. The frame comprises a single, unified steel casting which includes a plurality of circumferentially distributed struts extending radially between an inner hub and an outer case. The fairing assembly includes at least one fairing segment secured over a plurality of annular frame surfaces between the inner case and the outer hub, and defines a main gas flow passage through the frame.

A gas turbine engine frame comprises an outer case, an inner hub, and a plurality of struts distributed circumferentially around the frame and extending radially between the inner hub and the outer case. The outer case, the inner hub, and the plurality of struts are formed from a single unified steel casting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a cross-section of a gas turbine engine.

FIG. 2 shows a detailed cross-section of the engine including an embodiment of a case assembly with a sand cast frame, a fairing assembly, and a heat shield assembly.

FIG. 3A is a perspective view of an example casting for a sand cast frame.

FIG. 3B shows the frame after machining the example casting of FIG. 3A.

FIG. 4 is an axial section view of a case assembly taken across line 4-4 of FIG. 2.

FIG. 5 is a radial section view of a strut taken across line 5-5 of FIG. 3B.

DETAILED DESCRIPTION

The diameter of some gas turbine frames, including the inner and outer frame cases, can in some cases exceed 2 meters. The case can comprise a single cast steel frame to simplify manufacturing. Sand casting can be used to make the steel frame as a single, unitary, and monolithic piece. In certain embodiments, struts are cast solid, and passages for cooling and service tubes are machined radially through the struts after casting. Machining may be performed with high-speed milling equipment due to the resulting radial length of the passages. Fairings pass through the cast frame to define a main gas flow passage. Operating temperature of the frame can be reduced or maintained using a combination of sealing, internal cooling, external cooling, film cooling, and/or heat shields.

FIG. 1 shows industrial gas turbine engine 10, one example of a gas turbine engine. Engine 10 is circumferentially disposed about a central, longitudinal axis, or engine centerline axis 12, and includes in series order, low pressure compressor section 16, high pressure compressor section 18, combustor section 20, high pressure turbine section 22, and low pressure turbine section 24. In some examples, a free turbine section 26 is disposed downstream of the low pressure turbine 24. Free turbine section 26 is often described as a “power turbine” and may rotationally drive one or more generators, centrifugal pumps, or other apparatus.

As is well known in the art of gas turbines, incoming ambient air 30 becomes pressurized air 32 in compressors 16, 18. Fuel mixes with pressurized air 32 in combustor section 20, where it is burned. Once burned, combustion gases 34 expand through turbine sections 22, 24 and power turbine 26. High and low pressure turbine sections 22, 24 can drive respective high and low pressure rotor shafts 36, 38. Shafts 36, 38 can be rotated in response to the combustion products and in turn can rotate the attached compressor sections 18, 16. Free turbine section 26 may, for example, drive an electrical generator, pump, or gearbox (not shown) via power turbine shaft 39.

FIG. 1 also shows turbine assembly 40, which includes two turbine modules interconnected by a case assembly 42. Here, turbine assembly 40 can include turbine exhaust case (TEC) assembly 42 disposed axially between low pressure turbine section 24 and power turbine 26. However, it will be appreciated that case assembly 42 can be adapted to other interturbine cases requiring a frame.

FIG. 1 provides a basic understanding and overview of the various sections and the basic operation of an industrial gas turbine engine. Although illustrated with reference to an industrial gas turbine engine, the described subject matter also extends to aero engines having a fan with or without a fan speed reduction gearbox, as well as those engines with more or fewer sections than illustrated. It will become apparent to those skilled in the art that the present application is applicable to all types of gas turbine engines, including those in aerospace applications. In this example, the subject matter is described with respect to TEC assembly 42 between turbine sections 24, 26 configured in a sequential flow arrangement for an industrial gas turbine engine. However, it will be appreciated that the teachings can be readily adapted to other turbine applications with fluidly coupled modules, such as but not limited to a mid-turbine frame, an interstage turbine frame, and/or a turbine exhaust case for an aircraft engine. In other alternative embodiments, TEC assembly 42 can be adapted into a case assembly or module for portions of compressor sections 16 and/or 18.

FIG. 2 shows TEC assembly 42 which is adapted to interconnect an upstream turbine module 44 with a downstream turbine module 45. Upstream turbine module 44 (partially shown) can comprise as a low-pressure turbine module. Downstream module 45 (partially shown) can comprise as a power turbine module.

As shown in FIG. 1, low-pressure turbine 24 can drive a first shaft (low pressure shaft 38), while power turbine 26 can drive a second shaft (power turbine shaft 39) independently of the first shaft (low pressure shaft 38). In a conventional industrial gas turbine (IGT) system, upstream module 44 (e.g., low-pressure turbine 24 shown in FIG. 1) can include other components (not shown) such as a rotor blade and/or exit guide vane. These other components are disposed upstream of frame 46 and fairing assembly 48 with respect to a flow direction of working gases 34. Downstream module 45 (e.g., power turbine 26 shown in FIG. 1) can also include other components (not shown) such as an inlet guide vane and/or rotor blade. These other components are disposed downstream of TEC assembly frame 46 and fairing assembly 48 with respect to the conventional flow direction of working gases 34.

As seen in FIG. 2, TEC assembly 42 includes frame 46 and fairing assembly 48. Fairing assembly 48 can at least partially define main gas flow passage 51 for working/combustion gases 34 to flow generally axially through frame 46 during engine operation.

In the illustrated embodiment, frame 46 includes outer case 54, inner hub 56, and a circumferentially distributed plurality of struts 58 (only one shown in FIG. 2). Struts 58 extend radially between outer case 54 and inner hub 56. Frame 46 can be formed from a single steel casting as described in more detail below.

In the embodiment shown, fairing assembly 48, which includes outer fairing platform 60, inner fairing platform 62, and strut liners 64. Outer fairing platform 60, inner fairing platform 62, and fairing strut liners 64 define a portion of main gas flow passage 51. Outer fairing platform 60 and inner fairing platform 62 each have a generally conical shape secured over annular surfaces of outer case 54 and inner hub 56. Inner fairing platform 62 is spaced from outer platform 60 by strut liners 64, which are secured over surfaces of each strut 58 extending through main gas flow passage 51. In this example, outer fairing platform 60 is disposed radially inward of outer case 54, while inner fairing platform 62 can be disposed radially outward of inner frame hub 56.

Upstream (first) turbine module 44 includes outer case 70 connected to a forward side of outer case 54 via fasteners 72, while downstream (second) turbine module 45 includes outer case 74 connected to an aft side of outer case 54 via fasteners 76. Outer case 54 similarly includes forward flange 79A and aft flange 79B. TEC assembly 42 includes aft casing flange 79A and forward casing flange 79B for interconnecting TEC assembly 42 with other modules in engine 10 (shown in FIG. 1).

In addition, main gas flow passage 51 can be sealed around these and other interconnections to prevent fluid leakage and unwanted heating of frame 42. In one example, seals (not shown) are located around the edges 80 of fairing assembly 48. One or more of these seals may be part of a larger seal assembly (not shown) adapted to perform multiple sealing and support functions while helping to direct secondary air flow in and around frame 46.

TEC assembly 42 also can include heat shield assembly 82 comprising one or more heat shield segments 84. Heat shield assembly 82 reduces radiative heating of frame 46 by reflecting thermal radiation back toward fairing assembly 48 and away from annular surfaces of frame 46. Certain embodiments of heat shield assembly 82 also reduce convective heating to varying degrees, depending on whether one of more heat shield segments 84 are free to thermally grow.

Heat shield segments 84 are generally arranged in lines of sight between fairing assembly 48 and frame 46, but are not secured directly to the hottest portions of fairing assembly 48 designed to be exposed to working gas flow 34. Rather, heat shield segments 84 can be secured to cooler portions of TEC assembly 42 such as frame 46 or external fairing flanges 86 as shown in FIG. 2.

In the illustrated example, two heat shield segments 84 include a case portion parallel to respective outer and inner fairing platforms 60, 62. These two segments also can include radial extensions. Other segments 84 can include both axial and radial portions. One or more segments 84 can overlap. Overlapping segments can be fastened or welded together. Alternatively, overlapping segments can rest against one another and be free to thermally grow as needed.

Frame 46 can also include passages 90 (shown in phantom) formed radially through struts 58. To further reduce temperature of frame 46, at least one passage 90 can carry cooling air between outer cavity 92 and inner cavity 94. This cooling air can be used for convective cooling, film cooling, and/or impingement cooling of frame 46, fairing assembly 48, and/or heat shield assembly 82. Inner cavity 94 is disposed radially inward of inner hub 56, and is defined by inner hub 56, bearing support 96, and outer flow divider wall 98. As such, additional passages 90 may carry oil or buffer air service lines (not shown in FIG. 2) which continue through both inner cavity 94 and bearing support 96 into a bearing compartment (not shown).

These and other features of frame 46 allow for substitution of lower temperature materials and processes in place of more expensive temperature-resistant materials such as investment cast nickel-based superalloys. Here, frame 46 can be formed from a single-piece steel sand casting as described below.

FIG. 3A shows frame casting 114 prior to internal and/or external machining. Casting 114 also includes outer cast section 116, inner cast section 118, solid strut bars 120, and cast external features 122. FIG. 3B isometrically depicts frame 46, which includes a plurality of circumferentially distributed struts 58 extending radially between outer case 54 and inner hub 56.

Frame 46 (shown in FIG. 3B) can be produced by machining an example frame casting 114 as seen in FIG. 3A. To simplify manufacture and reduce material costs, frame casting 114 comprises steel with outer cast section 116, inner cast section 118, and solid strut bars 120 sand cast as a single steel piece. Bars 120 are generally box-shaped but can have one or more curved edges and/or junctions so as to improve castability and reduce defects. Sand casting is a cost-effective and repeatable process and can be adapted for producing large structural steel components. In sand casting, a sacrificial model of casting 114 is placed in a vat or other mold full of heated silica or other sand-like material. Molten steel is poured or injected into the mold in the vicinity of the model so that the molten steel takes the place of the wax, polystyrene, or other sacrificial material. The sand in the mold holds the molten steel in place and conducts heat away from the steel so that it can solidify into a casting.

Casting 114 can comprise a corrosion-resistant chromium steel with high thermal resistance and mechanical strength. In certain embodiments, the steel alloy comprises between about 11 wt % and about 14 wt % chromium, as well as about 3 wt % to about 5 wt % nickel. In certain of these embodiments, the steel further comprises up to about 1 wt % molybdenum. ASTM A743 class steel is one suitable non-limiting example in this range of compositions. More specifically, ASTM A743, Grade CA-6NM has been found to offer a suitable balance of castability, corrosion resistance, and thermal resistance among other factors.

In certain embodiments, sand casting 114 has a minimum radial dimension d measuring at least about 1.5 meters (about 59 inches). In certain of these embodiments, sand casting 114 has a minimum radial dimension d measuring at least about 2.1 meters (about 80 inches). These dimensions allow for a larger engine power core, and more efficient energy recovery from the downstream turbine module, such as power turbine 26 (shown in FIG. 1). Larger sand cast components such as frame 46 can be more cost-effectively produced as compared to the expense and labor required for investment cast superalloys. Investment casting of any alloy is made more difficult with components of this size.

FIG. 3A also shows external feature outlines 122, which form cast precursors to strut bosses 100, probe bosses 102, borescope bosses 104, and frame support stands 106 (shown in FIG. 3B). This saves time and effort spent on bulk machining as well as reducing waste. However, certain features shown in FIG. 3B such as struts 58 can be initially cast as solid strut blocks 118 (shown in FIG. 3A). In certain embodiments, this can provide a more repeatable thermal profile for solidification of casting 114, resulting in a lower rejection rate.

FIG. 3B shows a number of mounting, operational, and/or inspection features such as outer case mounting flanges 79A, 79B, strut bosses 100, probe bosses 102, borescope bosses 104, and frame support stands 106, which can be machined out of outer frame surface 108. They may be partially cast as shown in FIG. 3A, then finished machined into the form depicted in FIG. 3B. Other features such as cooling air inlets 110 and outlets 112 can be machined out through struts 58 as shown with reference to FIGS. 4 and 5.

FIG. 4 shows a cross-section of TEC assembly 42 taken across line 4-4 of FIG. 2. FIG. 4 illustrates an example cooling mechanism for cast frame 46. In this example, strut 58 includes cooling passage 90 formed radially therethrough. A plurality of film or showerhead cooling holes 123 are adapted to conduct a portion of frame cooling air from passage 90 to a periphery of strut 58 in order to reduce the temperature of one or more solid strut walls 124. FIG. 4 shows that cooling holes 122 conduct cooling air toward strut forward end 126. Additional cooling holes (not shown) can be adapted to conduct coolant toward strut aft end 128.

In this view, heat shield segments 84 are disposed around strut 58 between fairing strut liners 64 and outer strut surface 129. Cooling air can flow radially through one or both sides of heat shield segments 84

Passage 90 is defined by inner strut wall surface 130. In certain embodiments, portions of inner strut wall surface 130 can be shaped to accommodate one or more service lines 132. For example, inner strut wall surface 130 includes grooves 133 for larger service lines 132. FIG. 4 shows passage 90 as a single cavity. It will be appreciated that passage 90 can comprise multiple passages or cavities. Passage(s) 90 can be machined radially through strut 58 as explained with respect to FIG. 5.

FIG. 5 shows a radial cross-section of frame 46 taken across line 5-5 of FIG. 3B and includes passages 90 extending radially through struts 58 between inner hub 56 and outer case 54. When struts 58 are cast as part of frame casting 114 with solid strut bars 120 (shown in FIG. 3A), passages 90 must be machined radially. With increased radial casting dimensions d, traditional milling equipment can generate excessive heat and is prone to misalignment due to the length of each strut 58. For example, in castings measuring at least about 1.5 meters (about 59 inches), each strut 58 typically has a radial dimension s of at least about 0.5 meters (about 20 inches).

Thus in certain embodiments, passages 90 are formed radially through solid strut bars 120 (shown in FIG. 3A) using high-speed machining processes. These processes, sometimes known as “ballistic machining”, utilize specialized milling equipment to achieve high rotational tool speeds, along with cooling and chip removal features to precisely direct the tool through solid strut walls 124. Actual cutting speeds depend on such factors as the cutting tool material and size, as well as the ultimate tensile strength of the material. In the case of high strength steel alloys, rotational speeds can exceed 300 m/min. In certain of these embodiments, rotational speeds can exceed about 600 m/min.

While the machining equipment is more expensive and tooling life is relatively short, the combination of high-speed machining with a single sand cast frame provides a repeatable, cost effective alternative for large turbine frames as compared to investment cast or welded superalloys.

While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A gas turbine engine comprising: a first turbine module; a second turbine module; a frame interconnecting the first turbine module with the second turbine module, the frame comprising a single, unified steel casting which includes a plurality of circumferentially distributed struts extending radially between an inner hub and an outer case.
 2. The engine of claim 1, wherein the single unified steel casting has a minimum radial dimension measuring at least about 60 inches (about 1.5 meters).
 3. The engine of claim 1, wherein the single unified steel casting has a minimum radial dimension measuring at least about 80 inches (about 2.1 meters).
 4. The engine of claim 1, wherein the frame includes a radially extending cooling passage formed through each of the plurality of struts.
 5. The engine of claim 4, wherein the radially extending cooling passages are machined through solid cast struts.
 6. The engine of claim 1, further comprising: a fairing assembly including at least one fairing segment secured over a plurality of annular frame surfaces between the inner case and the outer hub, the fairing assembly defining a main gas flow passage through the frame.
 7. The engine of claim 6, further comprising: a heat shield assembly including at least one heat shield segment disposed between the at least one fairing segment and an annular frame surface.
 8. The engine of claim 1, wherein the first module comprises a low pressure turbine module and the second module comprises a power turbine module.
 9. A turbine exhaust case (TEC) assembly for a gas turbine engine, the TEC assembly comprising: a frame comprising a single unified steel casting, the frame including a plurality of circumferentially distributed struts extending radially between an inner hub and an outer case; and a fairing assembly including at least one fairing segment secured over a plurality of annular frame surfaces between the inner case and the outer hub, the fairing assembly defining a main gas flow passage through the frame.
 10. The assembly of claim 9, wherein the single unified steel casting has a minimum radial dimension measuring at least about 60 inches (about 1.5 meters).
 11. The assembly of claim 9, wherein the single unified steel casting has a minimum radial dimension measuring at least about 80 inches (about 2.1 meters).
 12. The assembly of claim 9, wherein the frame includes a ballistically machined cooling passage formed through each of the plurality of struts.
 13. The assembly of claim 9, wherein the single unified steel casting comprises at least about 11 wt % chromium and at least about 3 wt % nickel.
 14. A gas turbine engine frame comprising: an outer case; an inner hub; and a plurality of struts distributed circumferentially around the frame and extending radially between the inner hub and the outer case, the outer case, the inner hub, and the plurality of struts formed as a single unified steel casting.
 15. The frame of claim 14, wherein a radial dimension of the frame measures at least about 60 inches (about 1.5 meters).
 16. The frame of claim 14, wherein a radial dimension of the frame measures at least about 80 inches (about 2.1 meters).
 17. The frame of claim 14, wherein the single steel sand casting includes a plurality of solid cast strut bars.
 18. The frame of claim 17, further comprising: a radial cooling passage ballistically machined through each of the struts, each passage extending between the outer case and the inner hub.
 19. The frame of claim 18, further comprising: a cooling hole providing fluid communication between the passage and an outer surface of the strut.
 20. The frame of claim 14, wherein the single steel sand casting comprises at least about 11 wt % chromium and at least about 3 wt % nickel. 